We report the results of ac sheet conductance measurements performed on fully frustrated square arrays of Josephson junctions whose coupling energy is periodically modulated in one of the principal lattice directions. Such systems are predicted to exhibit two distinct transitions: a low-temperature Ising-like transition triggered by the proliferation of domain walls and a high-temperature transition driven by the vortex unbinding mechanism of the Beresinskii-Kosterlitz-Thouless (BKT) theory. Both the superfluid and dissipative components of the conductance are found to exhibit features which unambiguously demonstrate the existence of a double transition whose properties are consistent with the Ising-BKT scenario.

Crew Environments Branch, Brooks Air Force Base, Texas 78235Copper complexes have been evaluated for the detection of organophosphorus compounds In alr. Dlisopropyi methylphosphonatr (DIMP) was used as a model for the phosphorus esters. Bldentate copper complexes possess the hlghest activity for DIMP, and thls effectiveness was evaluated by chemically binding the complex to XAD-4, an experimental resln polymer. The calibration curve Is linear between 0 and 20 ppb DIMP (the range investigated). No signlflcant interference from automobile exhaust gas, chosen as a posslble pollutant, was observed.The purpose of this effort was to evaluate a series of copper complexes for the detection of organophosphorus compounds in air using ti coated piezoelectric quartz crystal. Piezoelectric crystals have been used as sorption detectors and microbalances, and several authors have discussed the theory and applications of this technology in the literature (1-8). In a previous effort concerned with detecting organophosphorus compounds, it was discovered that l-n-dodecyl-3-hydroxyiminomethylpyridinium iodide (3-PAD) was an adequate coating and that ita sensitivity could be increased by forming a ternary mixture with Triton X-100 and NaOH (8, 9).Several papers published on catalysts for hydrolysis of phosphorus esters cite the use of copper complexes; this fact suggesta that this class of compound is worthy of investigation. The most interesting study is from Wagner-Jauregg, et al. (IO).They found that Cu(I1) salts catalyze the hydrolysis of diisopropylphosphonate, and the catalytic effect is enhanced above pH 7. Copper has to be complexed to remain in solution, and lig,mds containing basic nitrogen were used for this purpose. The most effective catalyst found is the copper(I1) 2,2'-dipyridyl cation. Two monodentate or one bidentate ligand yield8 the optimal activity. With more ligand bonding, the activity (decreases markedly. Copper was found to be the best metal among tKose tested: nickel, cobalt, iron, and manganese. Since then, other workers have made use of copper as a catalyst for the hydrolysis of phosphorus esters (11-13). The overall reaction occurs in two steps: first, the copper complex binds the phosphorus ester reversibly; second, the adduct-product is irreversibly broken down by hydrolysis. The property of the first reaction is most usefulsin this effort. Copper complexes may adsorb and desorb phosphorus esters in air. The ,second reaction is unlikely to occur'on a copper complex used as a coating for a crystal, because of the low content of water in air; and thus, humidity is not expected to interfere.A significant portion of organophosphorus compounds contain either the phosphoryl or thiophosphoryl groups. As has been the practice in many previous studies concerning phosphoryl-containing compounds, diisopropyl methylphosphonate (DIMP) was used as a model compound in this effort (2, 8, 9, 14). EXPERIMENTAL SECTIONApparatus. The experimental instrumentation used to evaluate the coated piezoelectric quartz crystal detec...

The complex conductance G͑v͒ of site-diluted Josephson junction arrays close to the percolation threshold was measured over a wide range of frequencies v. Well below T c both the superfluid ͓vImG͔ and dissipative ͓ReG͔ components are independent of v below a critical frequency v c , whereas G͑v͒~v 2u with u ഠ

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The complexcs between different diorganotin cations and substituted 1,lO-phenanthrolines in aqueous solution containing acctatc ion have been investigated. Diorganotin cations can co-ordinate two acetate ions. One of thcsc niay be replaced by a 1.10-phenanthroline derivative. A comparison of the different cquilibrium constants shows that the acid hardness of diorganotin cations is markedly modified if the organic groups bound to thc tin are a,B-unsaturated.Ides composCs organostanniques forment avec des bases de LEWIS un grand nombre de Complexes dans lesquels le nombre de restes organiques fix& & 1'Ctain ne d6passe jamais trois. A 1'Ctat solide, 1'Ctain des complexes triorganostanniques posskde en g6nCral la coordinance maximale cinq, et celui des complexes mono-et diorganostanniques, la coordinance maximale six 121.Comme le font apparaitre de rCcentes compilations [2] [3], les r6actions d'Cquilibre de ces composCs en solution ont Ct C relativenient peu CtudiCes. Ceci est valable en particulier pour les rCactions en solution aqueuse oh de nombreuses rCactions d'hydrolyse apparaissent [4], qui compliquent sbrieusement l'interprktation des rksultats.Une des questions en discussion concernant ces complexes est de savoir si les orbitales d libres peuvent participer a la liaison coordinative. Ainsi REATTIE et MCQUILLAN [5] ont Ctabli que la constante de force de la liaison Sn-CI est considkrablement modi-fiCe lorsque (CH,),SnCI est complex6 avec une niol6cule de pyridine, alors que la longueur de la liaison reste pratiquement la mCme; aussi, ces auteurs concluent-ils une contribution appr6ciable d, -$, k la liaison Sn-Cl. En revanche, en comparant entreelles les constantes de stabilitC de complexes de (CH,),Sn2 + avec l'acide picolinique,